Apparatuses for joining plastic materials, and their methods of use, are well known in the art. Typically, two or more plastic pieces are heated so that the long chain molecules of the plastic material loosen as the material transitions to a liquid phase. When subsequently cooled, the long chain molecules intertwine and solidify, thereby fusing the two sheets together.
Various apparatuses and methods are known for welding plastics. For example, Hot Wedge welding applies heated elements directly to the plastic material, while Hot Air welding, as the name suggests, applies heated air to the material. It is also known to use a solvent to bond or otherwise chemically fuse plastic elements.
Also know in the art is the technique known as radio frequency (RF) welding, also referred to as high frequency or dielectric welding. Conventional RF welding involves pressing two or more plastic pieces between two conducting elements and passing high frequency electromagnetic waves, typically 13-100 MHz, therebetween. The alternating electromagnetic field excites the plastic's molecules and thereby generates heat within the material. The plastic material is also simultaneously compressed by the conducting elements, in addition to being heated by the RF field.
Canadian patents 2,334,353 and 2,269,756 (both to Franciso et al.) disclose RF welding devices for bonding thermoplastic pads to sections of carpet for use in automobiles. In addition, Canadian patents 2,067,681 (Lovin), 2,084,843 (Lovin), 2,162,039 (Vallot) and 2,426,938 (Cook) disclose RF welding devices for forming medical bags. Canadian patents 2,176,689 (Taylor), 2,203,677 (Lippman et al.), 2,368,006 (Miyama et al.), 2,464,387 (Schwetz) and 2,482,023 (Johnstone) all disclosure various other apparatuses involving RF welding.
RF welding is also commonly used to join large sheets of polyvinyl chloride (PVC), polyethylene or polyurethane in order to form larger panels. It is also possible to RF weld other polymers including nylon, PET, EVA and ABS resins.
Typically, a PVC sheet will be laid onto an extended metallic surface and a second sheet will be laid thereon in an overlapping manner. A metallic die is then pressed onto the overlapping section and the high frequency electromagnetic field is generated between the two metallic components. The die is typically formed from an elongate metal bar. The footprint of the die, i.e. the dimensions of the surface which presses against the PVC sheet, is typically around one inch in width and, depending on the size and type of equipment used therewith, can be four or five feet in length. As such, the RF welding apparatus will create welded areas roughly one inch wide and five feet long. Successive welds may be themselves overlapped in order to join sheets longer than five feet.
The advantages of RF welding over other forms of welding are known in the art. For example, Hot Wedge welding typically involves a portable welding unit which is run along two overlapping plastic sheets. This type of welding requires each joint be formed in a single pass and therefore the size of any resultant plastic panel is limited by the size of the area in which the welding is taking place. Furthermore, it is know that both Hot Wedge and Hot Air welding are not well suited to welding thinner materials, such as those having a thickness of 30 thousandths of an inch or less, wherein, for example, the heat source may be positioned very close to the material surface which can result in the rapid degradation of materials such as PVC.
In contrast, RF welding creates joints in a sequential, step-by-step process wherein a continuous welded joint may be formed by repeatedly overlapping successive RF welds. As such, while a 300 foot long factory will limit the length of a Hot Wedge welded panel to 300 feet, it is possible to form 900 feet long plastic panels in a similarly sized factory by RF welding.
RF welding can be used to assemble individual PVC sheets together to form larger panels. Such PVC sheets can be configured and joined to form curved surfaces for use, for example, in lining pools and the like. Such PVC sheets can also be used to provide water and moisture-proof barriers and liners for civil engineering projects such as waste containment and the like.
It is also known that many uses of welded plastic sheets, such as when used for civil engineering purposes, require testing of welded joints prior to installation.
One such test requires that the welded joint which joins two plastic sheets include two separate welded bands forming an air channel therebetween. Prior to installation, the strength and integrity of the weld is then tested by closing the longitudinal extremities of the air channel and injecting pressurised air therein via a needle. The specific pressure within the air channel and the length of time which it must withstand that pressure are pre-determined as a function of one or more factors, such as the temperature of the sheets. A sudden drop in pressure during testing would indicate a leak in the seal and hence a faulty weld.
However, creating such air channels typically requires that a conventional RF welding apparatus form two separate welds side-by-side, one after the other. Ensuring the consistency and alignment of these parallel welds can be very complex and time consuming.
It would therefore be advantageous to provide an apparatus for welding plastic material which incorporates an air channel into the welded joint.
It would further be advantageous to provide such an apparatus which is suitable for use on large plastic sheets, preferably in a factory setting rather than on-site, such that it may be operated in a controlled environment and the resultant welded panels may be delivered prior to installation as a finished testable product.